Many portable communication devices are able to communicate with a cellular network using circuit-switched communication channel. Furthermore, some portable communication devices are now being able to communicate with a cellular network using a packet-switched communication channel. An example of such a portable communication device is a GSM-compatible mobile phone capable of using the General Packet Radio Service GPRS.
Building up a circuit-switched connection reserves more resources from the network. For technical and economical considerations, the circuit-switched service has in some cases priority over the packet-switched service. Especially this is the case where the portable communication device is being paged, i.e. when there is a call terminating at the portable communication device. Since a portable communication device usually has one transceiver only, and because its processing capability is rather limited, it cannot have an active packet-switched connection simultaneously with a circuit-switched connection. For GSM and GPRS this is particularly true for so-called GPRS Class B Mobiles that are defined as being able to support either packet-switched or circuit-switched communication channels, but not both simultaneously. Therefore, the packet-switched connections that are open must in some cases be suspended when a portable communication device receives a paging request.
For example, if during an established GPRS Packet Data Protocol (PDP) context some time-sensitive information, e.g. information that is useful or valid for a limited time only, has been transmitted to or from the portable communication device, suspending the GPRS connection could then in many cases obsolete the information, thereby causing a need to re-start the positioning attempt and to regenerate and re-transmit the information.
In the Secure User Plane SUPL requirement specifications of the Open Mobile Alliance OMA, this problem has been solved (e.g. for WAP) by requiring that the mobile station must enable an alternative and different communication channel (or data bearer). SMS could be selected as the different communication channel. In this case, nevertheless, the application in the portable communication device would get more complicated since it should also include a different message format for the SMS case. Furthermore, the server in the network, especially connected to or located in the packet-switched core network, would get more complicated because it should support the use of at least two communication channels and it should be able to handle a response message coming on another communication channel.